The present invention relates to crimped bicomponent polyester fibers, formed of two different polyesters having different thermal shrinking properties and different mechanical behavior with respect to various mechanical and/or thermal treatments, especially treatments subsequent to shrinking or stretching operations.
Elastic crimped fibers of polymeric materials have already been obtained by mechanical processing, such as assembling two filaments which are twisted in the opposite directions (or torsion twisting), and then fixing the twist and untwisting the resulting multifilament.
Bicomponent fibers have been obtained by extrusion of at least two different polymers, having different shrinking potentials, which may be of substantially the same or quite different nature, through the same spinneret die opening. Such bicomponent fibers have a latent crimping ability which can be developed during subsequent processing steps.
The results obtained by the prior art in the field of polyester filaments made of a single polymer and crimped by conventional processes, such as the use of a heated sharp edge or false twisting, have never been satisfactory, as the crimping and elasticity disappear because of a creep of the material, and especially during those times when the filaments are subjected to tension in the manufacture of finished products, such as woven or knit cloths, or in the process of manufacturing finished goods from such finished products. Polyester polymers are known to have significant plastic flow so that the elasticity of the fibers, even when the fibers are composite fibers, rapidly decreases with fatigue.
The prior art has suggested that a crimped bicomponent fiber be made of a homopolyester and a copolyester made from diacids, at least one diacid being common to the two polymers, and a diol and one or more triols. For instance, U.S. Pat. No. 3,454,460 discloses a composite polyester fiber formed of polymethylene glycol terephthalate, and a copolymer of the polymethylene glycol terephthalate and of a terephthalate of another diol or of a polymethylene glycol isophthalate.
French Pat. No. 1,486,035 describes a composite polyester filament made of polyethylene terephthalate and polyethylene terephthalate crosslinked with trimethylol propane.
French Pat. No. 1,442,768 discloses a composite fiber capable of developing a helical crimp and which is stable to heat and to deformation. Among the components taught by this patent as suitable for the composite fiber are polyethylene terephthalate and polybutylene terephthalate. Such products have a slightly better crimp permanency than other known bicomponent polyester fibers.
The prior art fibers described above, although having elastic properties which are more permanent than those of textured fibers produced by conventional mechanical processes, are still not completely satisfactory for textile uses.